This invention relates to a seal means for electrolytic cells and a method of sealing electrolytic cells.
Electrolytic cells employing a separator and particularly cells used for the production of chlorine and alkali metal hydroxides by the electrolysis of aqueous solutions of alkali metal chlorides are generally of two types, the diaphragm-type and the membrane-type.
Generally, the membrane-type cell employs a sheet-like membrane of ion exchange material, such as those marketed by E. I. duPont de Nemours and Company under the trademark Nafion.RTM. and by Asahi Glass Company, Ltd. under the trademark Flemion.RTM.. Such cells are commonly of the flat plate or filter press-type having monopolar or bipolar electrode structures. U.S. Pat. Nos. 4,108,742 and 4,111,779 issued to Seko et al. illustrate the bipolar system electrolytic cell. Illustrative of another design is taught by Kenny in U.S. Pat. No. 4,137,144 and the patent references cited therein.
In the filter press membrane-type cell, it is typical to clamp or otherwise compress the membrane in sheet form between the sides of the frame members. In addition, it has been common practice to interpose a gasket between one of the frame members and the surface of the membrane to form a fluid-tight, i.e., liquid- and gas-tight, seal upon compression of the frames and gaskets, and prevent leakage of electrolyte from one cell compartment to another or the environment. This compression typically is applied manually or mechanically utilizing hydraulic rams or other types of pressure-applying apparatus to compress the electrode frames and the separating gaskets together. Obtaining a fluid-tight seal, however, is desirably done without damaging the membrane.
The gasket material normally employed between the membrane and an electrode frame member of an electrolytic cell is of a resilient material, such as rubber or an elastomer. Commercial bipolar membrane electrolyzers generally use ethylene-propylene (EPM) or ethylene-propylene-diene (EPDM) as gasket material between the membrane and electrode frames. However, the above materials tend to deform and expand outwardly as pressure is applied to the frames via the frame members. As the gaskets deform outwardly, certain separators which are in contact with the gaskets tend to stretch when they are pulled under the pressure of the outwardly deforming gaskets. This stretching of the separator or membrane beneath the gaskets employed on adjacent electrode frames can cause the membranes to break or tear when attempting to compress the frames into a fluid-tight cell. Furthermore, resilient gaskets require a higher compressive force to effect a seal which increases the risk of breaking or tearing the membrane.
Any tears or breaks in the membranes may reduce current efficiency during operation, greatly increasing electrical current usage while reducing the electrolytic operating efficiency of the cell. Too great a drop in current efficiency and/or electrolytic operating efficiency can require costly shutdown of the entire cell while the damaged membrane or membranes are replaced.
Custom made gaskets, flat sheets or O-ring EPDM gaskets, are usually made and installed in cells to minimize the damage to the membrane. However, these resilient gaskets will still substantially recover their size and shape after release of a compressive stress applied to them. Thus, a resilient gasket cannot be pre-compressed and the membrane must be installed with the gasket between the cell frames before compression. This increases the likelihood that the membrane will be damaged when being compressed with the resilient gasket.
In view of the foregoing problems, it is desired to provide a seal means for electrolytic cells and a method for sealing the cells with such a seal means without damaging the membrane of the cell.